Fiber and integrated optic devices are being rapidly developed into components for use in waveguide circuits. Such optical waveguide circuits are usually characterized by a dielectric medium which carries electromagnetic radiation, usually in the optical spectrum along predetermined paths or conduits. These dielectric waveguide conduits are surrounded by a second dielectric medium with dielectric properties adjusted to cause electromagnetic radiation propagating through the waveguides to remain within such waveguides. For fiber optic devices this second medium takes the form of the cladding immediately surrounding the conducting fiber. For integrated optics devices, the substrate material and the air over the substrate usually serves as the second medium.
In the field of interferometry, heavy use of fiber optics and integrated optic devices is becoming common. An example is the Sagnac Interferometer wherein rotation rates about a given axis are accurately measured. A Sagnac Interferometer is pictured in FIG. 1 where source 60 directs light through fiber coupler 64 and waveguide 66 onto integrated optic chip 68, which contains Y-junction 72. The Y-junction splits the light beam into two beams which will traverse loop 70 in counter-propagating directions. It is the rotation about an axis perpendicular to the plane of loop 70 that is to be measured.
A rotation of the loop causes a change in phase between the counter-propagating beams. When the beams recombine at Y-junction 72, they are propagated back along waveguide 66 and are coupled down to detector 62. Detector 62 senses the intensity change resulting from the phase shift occurring in the combined beams and registers such as a measure of the rotation or rotation rate of the interferometer.
Recent work on such interferometers has utilized fiber optic components throughout, i.e. the integrated optic device 68 containing Y-junction 72 would be a second fiber optic coupler of the type depicted at 64. Such coupler would split the incoming beam of light and then recombine it after propagation through loop 70.
The desire to replace fiber optic devices with integrated optic devices has been fueled by the anticipation of better miniaturization capability as well as lower costs in producing such devices.
A preferable configuration for a Sagnac Interferomter is shown in FIG. 2. In this figure fiber optic coupler 64 and waveguide segment 66 have been replaced by integrated optics component 30. Integrated optics component 30 is configured with two Y-junctions 40 and 42, and connecting waveguide segment 41. Source 34 and detecter 36 are directly attached to integrated optics chip 30 at the respective legs of the first Y-junction 40. The second Y-junction 42 functions as the prior Y-junction 72 in FIG. 3 by splitting the input electromagnetic beam into counter-propagating beams in fiber loop 32. The returning counter-propagating beams are recombined by junction 42. The combined beams are then returned along waveguide 41 to detector 36 through the first Y-junction 40.
Many active or passive functioning components may be built into integrated optic chip devices such as 30. For example, polarizer 38 is shown built-in across waveguide segment 41, and modulating device 48 is shown built-in on the outbound leg 46 of the second Y-junction 42. Such elements are necessary to adjust polarization and modulation factors on the electromagnetic or light beams propagating the optical waveguides.
The obvious beneficial factors of using an optical chip 30 with two Y dividers has been impossible to achieve because of a known problem with radiation leakage into the substrate from the Y-junctions. FIG. 3 shows an integrated optic chip 10 with a double Y-junction waveguide constructed upon it. Considering conduits 16 and 18 as input waveguide legs to Y-junction node 12, we see that light coming in along one or the other of these conduits is joined and forced to continue along a single connecting waveguide conduit 28 to a second Y-junction mode 14. At node 14 the beam is split into separate beams to propagate out legs 22 and 20.
The problem occurs primarily at Y-junction node 12 where light is radiated from the junction into the substrate. This radiated energy is generally directed away from the waveguides at a small angle and would normally continue into the substrate material of integrated optic chip 10.
However, a small but significant portion of this radiated energy 24 is coupled back into those waveguide portions downstream from Y-junction node 12. Prior research has shown that this energy 26 re-enters the waveguide configurations along waveguide element 28, at Y-junction node 14, and in both of the waveguide conduits 20 and 22.
A technical analysis shows that light entering either of the single mode waveguide conduits 16 or 18 is composed of a symmetric mode and an antisymmetric mode of energy. At Y-junction node 12, the symmetric node is allowed to continue propagation within waveguide conduit 28, but the antisymmetric mode is stripped away and caused to radiate into substrate 10. In FIG. 3, then, stray radiation 24 represents this antisymmetric mode energy.
This phenomenon has been well presented in prior U.S. Pat. No. 4,468,085 by Papuchon, et al. and in the Article "RECIPROCITY PROPERTIES OF A BRANCHING WAVEGUIDE" by H. J. Arditty, M. Papuchon, and C. Puech, pp102-110, FIBER-OPTIC ROTATION SENSORS AND RELATED TECHNOLOGIES, edited by S. Ezekial and H.J. Arditty, Springer-Verlag, 1982.
The phenomenon results in bias errors of hundreds of degree/hours in Sagnac circuits and therefore makes such an integrated optic double Y-structure architecture unsuitable for use. This error source strongly limits accuracies desired to be achieved in interferometric applications. The present invention provides a concept and device which eliminates or greatly reduces this error.